Revell/Monogram has recently reissued its 1/72 B-52 with X-15. It was originally issued back in the '90s (I believe) in the "Young Astronauts" series, and for a time was sought after by collectors. The reissue made it much more available (and less expensive). Realistically, you'll be spending a lot of time creating your own carrying pylon and other mods to the bomber, and you can save yourself a lot of time (and trouble) if you find the B-52/X-15 issue.